In an internal combustion engine provided with a supercharger of an exhaust turbo type which has been recently developed, a turbine impeller receives the flow of exhaust gas, causing a compressor impeller to be rotated by the energy of the exhaust gas, so that an intake air is, under a supercharged condition, introduced into the engine. Thus, an increased amount of air is introduced into the engine when a throttle valve is fully opened. Thus, a necessary increase in the output power of the engine is attained during a full load of the engine.
In the conventional turbo system, the supercharging is always effected irrespective of a degree of throttle opening. However, the supercharging is not necessary during a partial load condition of the engine, since the throttle valve is, at this condition, moved from the fully opened condition in order to decrease the amount of intake air. Thus, the prior art suffers from a drawback in that energy is wasted in the compressor when the engine is operating under a partial load condition.
Supercharging during a partial or low load also causes a drawback in that drivability becomes worse during rapid acceleration or warming up because of the following reason. When an acceleration is effected by a rapid opening of the throttle valve, the rotational speed of the engine becomes high at the instant of the acceleration due to a pressure increase generated between the compressor and the throttle valve. However, an increase in the rotational speed of the impeller is slowed down due to the inertia of the impeller. Thus, a slow down of the increase of the rotational speed of the engine temporarily takes place, causing the drivability to deteriorate. When the engine is decelerated from an acceleration or high speed running, a decrease in the intake vacuum is delayed due to the inertia of the impeller, which adversely affects engine braking.
When a cold engine is started, an increase in torque necessary for operating the impeller takes place due to the viscosity of lubricant, so that the impeller is substantially not rotated. Thus, an intake resistance takes place to decrease the speed of increase in the rotational speed of the engine, causing engine to stall.
In order to overcome the above disadvantages a system has been heretofore presented for cancelling the operation of the supercharger when the engine is under a partial load. In such system, there is provided a by-pass passageway for introducing intake air into the engine without passing the compressor, or a by-pass passageway for preventing the exhaust gas from being introduced into the turbine to decrease turbine work. A combination of such by-pass passageways is also presented. These prior arts have as their object to decrease the back pressure and the temperature of the intake air, to improve the engine performance. The provision of the by-pass passageway, in the intake system requires extra parts, such as by-pass pipes and a valve, which additions are disadvantageous from the point of view of space and cost. The provision of the by-pass passageway in the exhaust system or combined by-pass passageways is disadvantageous in that the rotational speed of impeller becomes low so that the delay in the response of the supercharger is lengthened during transient conditions of the engine, such as acceleration or deceleration, cause the drivability to become worse.